I can define users' file permissions through chmod 777 (aka all rwx). However, how do we define permissions for 'Groups'? How is 'Groups' normally used in the security aspect? What is 'Groups' 's purpose?

Thanks !

chmod -- This changes the permission of files,not only for users but also to groups
Better you look at the man page for chmod you will get an idea

Suppose you have a staging server. You have 20 users on that box but only 5 of them have access to modify files and whatnot in a certain directory (let's just call the directory "pre_live"). You give the permissions to "pre_live and all files/subfolders" as 775 (rwx-rwx-rx) and as group "staging". You put Joe,Jane,Mary,Steve,and Rick in group staging. That means now, those 5 people are the only ones that can write/modify files in that directory while all others can just execute/read the files. That's how groups work.

-twantrd

If "pre_live and all files/subfolders" have permissions 755 (rwx-rx-rx), will these 5 people have rwx permission or rx permission, assuming they are the owners (rwx permission) of the files and belonging to the same group (rx permission)?

Thanks !

Instead of using numbers you can use letters: chmod u+rwx ..... gives all permission to the user. chmod g+rx gives read/excecution permissions to a group. chmod ug-w removes writting permissions for user and group, and so on...

If I have a file (payroll) with the following permissions and ownership:

-rwxrw-r-- 1 John Accts 100 2005-11-05 07:50 payroll

and John belongs to Accts group, would he have rwx or rw permission on the file?

Thanks for your patience

rwxrw-r--
Where
rwx is for owner of the file(user who owns it)
rw- is for other users of group
r-- is for those who are not in thefiles group
So john is the owner of the file and he have the permission rwx